In the lithography technique for forming wiring patterns and so forth, following the fact that formation patterns have been becoming much finer, it has become difficult to form the patterns using a light lithography technique being a conventional general-purpose technique, and therefore, an exposure technique using charged particle beams such as electron beams or ion beams, or short wavelength beams of an X-ray source or the like has been positively discussed for achieving further fineness. Among them, an electron beam writing technique has been fostered such that there have been proposed a variable-shaped writing method which, starting from the initial spot beam writing, carries out writing by changing the size and shape of rectangular beams, and subsequently, a partial batch writing method that writes part of a pattern partially in a batch via a mask and repeats it, in terms of improving pattern accuracy, shortening a writing time, and so forth. Then, subsequently to the partial batch writing method, a new electron projection system (SCALPEL system) was proposed by S. D. Berger, et al. about eight years ago. Thereafter, there have been various proposals about similar writing systems (PREVAIL systems), and transfer mask (reticle) structures for application to these writing systems and production methods thereof.
For example, in the PREVAIL system invented by H. C. Pfeiffer, et al., in brief, preparation is made of a stencil mask formed with a through hole (aperture) pattern that is formed with predetermined size and arrangement in each of small regions, then charged particle beams are irradiated onto the small regions so that beams formed via the through hole patterns are applied via an optical system onto a to-be-exposed substrate formed thereon with a photosensitive member to thereby transfer the through hole patterns in a reduced size on the substrate, so that a device pattern is formed while joining together on the exposed substrate the predetermined patterns that are dividedly formed on the mask (see Publication of Pat. No. 2,829,942). The transfer mask proposed for this system is mainly configured by a stencil type mask with pattern portions comprising through holes not shielded at all (see Laid-open Unexamined Patent Publication No. Hei 10-261584 and Laid-open Unexamined Patent Publication No. Hei 10-260523). In the stencil type mask, by dividing and reinforcing a pattern region from the back by the use of a strut (frame) structure, reduction in deflection of the pattern region is achieved to thereby achieve improvement in pattern position accuracy, and so forth.
On the other hand, as a mask structure for the SCALPEL system, a scattering mask (reticle) has been mainly proposed rather than the stencil mask (see Laid-open Unexamined Patent Publication No. Hei 10-261584). According to the description thereof, the mask structure is such that a heavy metal layer is formed on a membrane (self-standing thin film) of SiN or the like, and predetermined pattern formation is applied to the heavy metal layer. Although electron beams are irradiated onto both layers, the electron scattering degree differs according to the presence or absence of electron beam scatterers. It is a method of obtaining beam contrasts on a wafer utilizing this difference in scattering degree to thereby carry out transfer of patterns in a reduced size.
These exposure systems each satisfy a high resolution being a feature of the charged particle beam and enable pattern formation finer than 0.1 μm and, when compared with the partial batch method, achieve improvement in throughput in the production of devices (e.g. throughput of 30 sheets or more per hour with the minimum line width of 0.08 μm and 8-inch substrates) due to large magnification of a shot size (e.g. magnification of the maximum shot size from 5 μm to 250 μm on an exposed substrate) and so forth, and have a machine capability of dealing with production of general-purpose devices, and therefore, are highly practical systems.
In the lithography masks as described above, the position accuracy of the mask patterns is very important in terms of a registration accuracy of transferred patterns. Particularly, when the patterns are formed in the self-standing thin film, there has been a problem that the pattern position accuracy is easily lowered due to influence of a film stress thereof.
With respect to such a problem, there has been proposed a method of rendering a stress of a self-standing thin film as small as possible in a tensile direction, and selecting a material providing a small film stress of a photosensitive member (resist) used for pattern formation or achieving reduction of a stress of a resist by a heat treatment condition (see S. Tsuboi, et al. Jpn. J. Appl. Phys. 35. 2845 (1996)).
However, in the method of rendering the stress of the self-standing thin film as small as possible in the tensile direction and reducing the stress of the resist, the resist stress can not be reduced to zero, and therefore, there has been a limit as a measure for solution. Further, there has been a problem that although the self-standing thin film to be formed with patterns is normally required to have a minimum tensile stress for ensuring its self-standing property, the continuity of the self-standing thin film is reduced due to the pattern formation in the self-standing thin film, and therefore, the stress of the self-standing film changes unavoidably before and after the pattern formation. To this end, it has been difficult to produce a mask so as to stably obtain a high position accuracy (10 nm or less).